This invention relates to emission spectroscopy and more particularly to an apparatus for emission spectrochemical analysis which utilizes spark discharge as the light source and is suitable for quantitative analysis.
Emission spectrochemical analysis is a method by which the elements contained in a sample are determined by exciting the sample to produce light, dispersing the light to produce a spectrum and identifying the element causing each of the emission lines in the spectrum.
To conduct a quantitative analysis by this method, both a sample to be analyzed and a reference which is of the same kind as the sample to be analyzed and which contains the elements to be quantitatively determined in known percentage are provided so that the sample and the reference are separately excited to obtain the spectral lines of the elements being determined, and the amount of each of the elements in the sample is determined by the ratio of the intensity of the spectral line of each of the elements in the sample being analyzed to the intensity of the corresponding spectral line of the same element in the reference.
The above method, however, has the disadvantage that it is time-consuming since a separate analytical operation is performed on the sample and the reference, and that since the two operations are conducted separately, strictly the analytical conditions are different with resulting poor reproducibility of the results of analysis.
To overcome the disadvantage, a method called the internal standard method has been proposed for quantitative determination by emission spectrochemical analysis. This method depends on a particular element always present in the sample in known amount and referred to as the internal standard element. When the sample is excited, the intensity of the spectral emission line produced by the particular element (to be referred to as the internal standard line) and that of the emission line produced by the element to be quantitatively determined are measured so as to obtain the intensity ratio therebetween. Separately, several references of the same kind as the sample to be analyzed are prepared which contain the element to be determined in different known amounts, and the intensity ratio of the emission line of the element to be determined to the internal standard line is measured for each of the references so as to plot the intensity ratio against the content of the element to be determined to obtain a calibration curve. Then by locating the intensity ratio of the spectral emission line obtained from the sample being measured to the internal standard line on the calibration curve to see what amount of the element the location of the ratio on the curve corresponds to, it is possible to determine the element quantitatively.
According to this method, quantitative comparison of the element to be determined and the internal standard element is made in the same sample and by the same procedure with resulting improvement in the reproducibility of measurement over the previously mentioned method. Moreover, a single operation suffices for analysis. With a calibration curve once plotted, it is not necessary to prepare a new curve for each operation of analysis.
When emission spectrochemical analysis is conducted on a test piece cut from a solid sample such as metal, spark discharge is suitably employed as the light source, and in this case the internal standard method is also employed for quantitative analysis. If carbon, silicon, manganese, sulfur, etc. in, for example, steel are to be quantitatively determined, the base metal, that is, iron itself is used as the internal standard element, and a suitable one of the emission lines of the spectrum produced by iron is used as the internal standard line.
The invention provides an apparatus for emission spectrochemical analysis which employs spark discharge as the light source and which is suitable for quantitative determination by the internal standard method.
Spark discharge comprises repeated pulse-like discharges, and in order to improve the precision of measurement it is customary to continue the discharge for a period of time, so that the light emitted during the period is converted to corresponding electric current for integration thereof. In practice, discharge is repeated at the rate of about 400 times per second and the values measured at individual discharges are integrated for a period of several to several tens seconds to obtain the average of the measured values. In quantitative analysis by the internal standard method spark discharge is caused to occur several thousands of times, at each of which the intensity of the internal standard line and that of the emission line of the element to be quantitatively determined are measured, and the measured values are averaged for the internal standard as well as for the element to be determined to calculate the ratio between the two averages, from which the content of the element to be determined is obtained with reference to a calibration curve.
Spark discharge, however, is a phenomenon so unstable that the state of light emission varies greatly each time discharge occurs. As a result, even with the above-mentioned averaging operation and despite the good reproducibility that the internal standard method inherently has, the conventional method of emission spectrochemical analysis utilizing spark discharge has been found unsatisfactory with respect to reproducibility.
Accordingly, the object of the invention is to provide an apparatus for emission spectrochemical analysis which utilizes spark discharge as the light source and which has a good reproducibility of the results of quantitative analysis.
The invention will be explained in detail with reference to the accompanying drawing.